1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to a wavelength division multiplexing passive optical network.
2. Description of the Related Art
A wavelength division multiplexing (WDM) passive optical network distributes, optical signals downstream from a central office to each subscriber. The network uses at a remote node between the central office and the subscriber a passive element instead of an active element, such as an amplifier, a transmitter and so forth. Similarly, upstream optical signals together with data for the subscriber are sent through the passive element to the central office. In such a WDM passive optical network, both the central office and the remote node require an apparatus for multiplexing and/or demultiplexing upstream and downstream optical signals, and both the central office and the subscriber require a transmitter and a receiver.
A persistent attempt has been made in recent WDM passive optical networks to efficiently implement a multiplexer and a demultiplexer for use with an arrayed waveguide grating (AWG) and to economically implement a light source for transmission. In particular, much research has recently been published regarding a method for implementing the light source for transmission of the upstream optical signals in a simple and inexpensive manner, in which the light source is provided for the subscriber.
FIG. 1 illustrates construction of a conventional WDM passive optical network. This WDM passive optical network includes a central office 110 and a subscriber-side apparatus 130,140 connected with the central office 110 through optical fiber links 120, 125, respectively. Here, the subscriber-side apparatus includes a remote node 130 and a plurality of subscribers 140 connected with the remote node 130.
The central office 110 includes a plurality of optical transmitters 111, a plurality of optical receivers 114, a multiplexer 112 and a demultiplexer 113.
Each of the optical transmitters 111 outputs channels, which are modulated with the corresponding data signals and have different wavelengths. The optical transmitter 111, for example, makes much use of a distributed feedback laser diode (DFB LD) appropriate to downstream optical signals for transmission of high capacity data.
The multiplexer 112 multiplexes channels inputted from the transmitters 111 into downstream optical signals, and then transmits the multiplexed downstream optical signals through the optical fiber link 120. The multiplexer 112 may make use of a passive element such as a 1×N AWG, for example.
The demultiplexer 113 demultiplexes upstream optical signals received through the optical fiber link 125 into a plurality of channels, and then outputs the demultiplexed channels. The demultiplexer 113 may also make use of a passive element such as a 1×N AWG.
The optical receivers 114 convert each of the channels inputted from the demultiplexer 113 into an electrical signal. The optical receiver 114 makes use of a photodiode, for example.
The subscriber-side apparatus 130, 140 includes a demultiplexer 131, a multiplexer 132, a plurality of optical transmitters 142 and a plurality of optical receivers 141.
The demultiplexer 131 demultiplexes downstream optical signals received through the optical fiber link 120 into a plurality of channels, and then outputs the results.
The optical receivers 141 convert each of the channels inputted from the demultiplexer 131 into an electrical signal.
The optical transmitters 142 each output a plurality of channels, which are modulated with the corresponding data signals and have different wavelengths. The optical transmitter 142, for example, makes much use of either a spectrum-sliced light source appropriate to upstream optical signals for transmission of relatively low capacity data or a light source for generating only a single mode by locking a multi-mode laser diode.
The multiplexer 132 multiplexes a plurality of channels inputted from the transmitters 142 into upstream optical signals, and then transmits the multiplexed upstream optical signals through the optical fiber link 125.
However, the conventional WDM passive optical network has a problem in that transmission of a high volume of optical signals caused by a sharp growth in volume of the upstream optical signals, when either the spectrum-sliced light source or the light source for generating only a single mode by locking a multi-mode laser diode is used as a light source for transmitting upstream optical signals, makes operation of the network impossible or causes a transmission error rate to increase rapidly. As a further problem, costs for constructing the subscriber-side apparatus are increased when a single mode laser diode such as the DFB LD is used as a light source for transmitting upstream optical signals.